To use an album from your Photos Library as a screensaver, you have to select in the System Preferences as the source of the Screensaver. After selecting a slideshow screensaver select a source.
This is described here: Change Screen Saver preferences on Mac – Apple Support (UK)
One caution: The most recent versions of macOS are not showing the edited versions in the screensaver, just the originals. I don't remember, if this already appies to Big Sur. If your photos are having adjustments applied or cropping and rotation,but the screensaver is only showing the originals, export the album to a folder in the Finder and use the exported album as the source of the screensaver.